Hi sonjaandsue,
If you have already bought plane tickets then I would definitely go ahead and book a hotel. In fact, I would book two hotels rooms, one at the Disney hotel of my choice and another at a less expensive off-site hotel as a back up.
When you make a reservation with the Walt Disney Travel Company, you will be required to pay one night in full as a deposit. However, Disney hotels have a 5 day cancellation policy. You can cancel up to 5 days before your reservation and get a full refund. Keep in mind that this is for room only reservations. The rules are different for packages. You can also rebook your room at a lower rate if a fall special comes out after you have booked your room.